The supplying of clock information in an Audio Video Bridging (AVB) network is based on the recovery of a clock from presentation timestamps that are received in a media stream. Real time transporting of digital audio and video and other digital media over data networks creates a new set of problems compared to non-media data. For instance, data networks may use packet switching in which data is divided into packets for separate transmission. As the packets are transmitted, sequential packets may take different routes and include different transit times. The packets are numbered to ensure they can be reordered correctly after arrival. This technique, however, does not suffice when left and right audio signals are to be received at different destinations, for instance at different speakers.
Unlike non-media data, digital media must be played out in synchronization. For example, video and audio must be aligned in time so that when they are played out the images match the sound.
The concept of a network clock has been used to address timing problems in data networks. A network clock signal is typically generated at a specific point in the network and this becomes a system time signal received by devices on the network. The system time signal is then used as a time reference for every device that receives the system time signal. Because of the topology of the network, devices at different locations on the network will receive the clock signal with a phase offset from the network clock, depending on the propagation delay from the clock to the device. A further consequence is that different remote devices may include received clock signals that include phase offsets with respect to each other, as well as with respect to the network clock.
Digital media transmission has historically embedded clocking information in the transmitted data. Embedding and recovering clocking information from data signal transitions or packet timing (e.g., Audio Engineering Society standard AES3, Sony/Philips Digital Interface, Gibson Media-accelerated Global Information Carrier) works well for point to point links between a small number of devices, but as the number of devices increases, clock jitter cascades and builds through each device that recovers and re-transmits the clocking information.
Digital media transmissions may alternatively employ a Time Division Multiplexing (TDM) approach. In TDM systems (e.g. Multichannel Audio Digital Interface, CobraNet), a master clock device initiates periodic transmission cycles and each device is allocated one or more time slots within that cycle for transmission. This limits the total available number of channels.